Fuel cells have been proposed as a power source for many applications. So-called PEM (proton exchange membrane) fuel cells [a.k.a. SPE (solid polymer electrolyte) fuel cells] potentially have high energy and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a "membrane electrode assembly" (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The membrane-electrode-assembly is sandwiched between a pair of electrically conductive elements which serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. The channels/openings for the reactants are often referred to as "flow channels". A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
PEM fuel cells are typically H.sub.2 --O.sub.2 fuel cells wherein hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., O.sub.2), or air (i.e., O.sub.2 admixed with N.sub.2). The solid polymer membranes are typically made from ion exchange resins such as perfluoronated sulfonic acid. One such resin is NAFION.TM. sold by E. I. DuPont deNemeors & Co. Such membranes are well known in the art and are described in U.S. Pat. No. 5,272,017 and 3,134,697, and in Journal of Power Sources, Volume 29 (1990), pages 367-387, inter alia. The anode and cathode themselves typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles and proton conductive resin intermingled with the catalytic and carbon particles. One such membrane electrode assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention.
The hydrogen used in the fuel cell can be derived from the reformation of methanol or other organics (e.g., hydrocarbons). Unfortunately, the reformate exiting the reformer contains undesirably high concentrations of carbon monoxide which can quickly poison the catalyst of the fuel cell's anode, and accordingly must be removed. For example, in the methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to the reaction: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2
This reaction is accomplished heterogeneously within a chemical reactor that provides the necessary thermal energy throughout a catalyst mass and actually yields a reformate gas comprising hydrogen, carbon dioxide, carbon monoxide, and water. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh. Carbon monoxide (i.e., about 1-3 mole %) is contained in the H.sub.2 -rich reformate/effluent exiting the reformer, and must be removed or reduced to very low nontoxic (i.e., to the anode) concentrations (i.e., less than about 20 ppm) to avoid poisoning of the anode by adsorption onto the anode catalyst. The unreacted water serves to humidify the fuel gas and prevent drying of the MEA.
It is known that the carbon monoxide, CO, level of the reformate/effluent exiting a methanol reformer can be reduced by utilizing a so-called "shift" reaction. In the shift reactor, water (i.e. steam) is injected into the methanol reformate/effluent exiting the reformer, in the presence of a suitable catalyst, to lower its temperature, and increase the steam to carbon ratio therein. The higher steam to carbon ratio serves to lower the carbon monoxide content of the reformate according to the following ideal shift reaction: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
Some CO survives the shift reaction and remains in the reformate. Depending upon the reformate flow rate and the steam injection rate, the carbon monoxide content of the gas exiting the shift reactor can be as low as 0.5 mole %. Any residual methanol is converted to carbon dioxide and hydrogen in the shift reactor. Hence, shift reactor effluent comprises hydrogen, carbon dioxide, water and some carbon monoxide.
The shift reaction is not enough to reduce the CO content of the reformate enough (i.e., to below about 20 ppm). Therefore, it is necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the shift reactor, and prior to supplying it the fuel cell. It is known to further reduce the CO content of H.sub.2 -rich reformate exiting the shift reactor by a so-called "PROX" (i.e., preferential oxidation) reaction effected in a suitable PROX reactor and can be either (1) adiabatic (i.e. where the temperature of the catalyst is allowed to rise during oxidation of the CO), or (2) isothermal (i.e. where the temperature of the catalyst is maintained substantially constant during oxidation of the CO). The PROX reactor comprises a catalyst bed operated at temperatures which promote the preferential oxidation of the CO by injecting controlled amounts of air into the effluent from the shift reactor to consume the CO without consuming/oxidizing substantial quantities of the H.sub.2. The PROX reaction is as follows: EQU CO+1/20.sub.2 .fwdarw.CO.sub.2
Desirably, the O.sub.2 required for the PROX reaction will be about two times the stoichiometric amount required to react the CO in the reformate. If the amount of O.sub.2 is substantially less than about two times the stoichiometric amount needed, insufficient CO oxidation will occur. On the other hand, if the amount Of O.sub.2 exceeds about two times the stoichiometric amount needed, excessive consumption of H.sub.2 results. Consumption of the H.sub.2 raises the temperature of the gas, which in turn causes the formation of CO by the reaction of H.sub.2 with CO.sub.2, known as the reverse gas-shift reaction. Hence, careful control of the amount of air injected in the PROX reaction is essential to control the CO content of the reformate feed stream to the fuel cell. The PROX process is described in a paper entitled "Methanol Fuel Processing for Low Temperature Fuel Cells" published in the Program and Abstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach, Calif. and in U.S. Pat. No. Vanderborgh et al 5,271,916, inter alia.
Whether an adiabatic or isothermal PROX reaction, a controlled amount of O.sub.2 (i.e., as air), is mixed with the reformate exiting the shift reactor, and the mixture passed through a suitable PROX catalyst bed known to those skilled in the art. To control the air input rate, the CO concentration in the gas exiting either the shift reactor or the PROX reactor is measured, and based thereon, the O.sub.2 concentration needed for the PROX reaction adjusted. However, sensitive, real time, CO sensors have not heretofore been available, and accordingly system response to CO concentration variations has been slow. This is particularly troublesome in dynamic systems where the flow rate, and CO content, of the H.sub.2 -rich reformate vary continuously in response to variations in the power demands on the fuel cell system. Since the amount of O.sub.2 (e.g., air) supplied to the PROX reactor must vary on a real time basis in order to accommodate the varying power demands on the system, there is a need for a rapid response CO sensor to continuously monitor the CO in the reformate stream and therefrom (1) maintain the proper oxygen-to-carbon monoxide concentration ratio in the PROX reactor, and/or (2) divert the reformate stream away from the fuel cell until the CO content thereof falls within acceptable levels.